A variety of hangers have been employed to support workpieces such as automotive parts in electrocoating processes. In a typical electrocoating procedure, one or more workpieces made of an electrically conductive metal are suspended on an electrically conductive hanger lowered into a bath containing a charged paint. An electrical current is caused to flow through the hanger and workpieces so that the charged paint adheres strongly and coats the workpieces. After the workpieces have been dipped in the charged paint for a suitable time, they are rinsed to recover unused paint, and then dried at an elevated temperature. The hanger or rack used in such a conventional electrocoating process typically comprises a support frame having a number of arms (pins) or hooks mounted thereto for supporting a number of workpieces. Such a hanger is typically lowered gradually into the charged paint bath while being conveyed horizontally by an overhead conveyor system.
Workpieces of certain peculiar shapes present problems during electrocoating. For example, a cylindrical can is particularly difficult to hang in a position suitable for electropainting. If the can is hung so that it tilts downwardly (relative to the horizontal) there will be an air pocket inside the can when it is lowered into the paint bath. This will normally prevent a portion of the inner surface of the can (at the location of the air pocket) from being painted. If the can is hung on the hanger so that it tilts upwardly, an undesirable pool of paint will remain in the can after it emerges from the paint bath. If the can is held dead flat, i.e., precisely horizontal, such problems with drainage and air pockets can in theory be avoided. In practice, however, it is difficult and inconvenient to provide a hanger which holds workpieces horizontal. In some cases, even where the workpiece is held horizontal, foam spots develop on the side of the can-shaped workpiece which is lying horizontally. Such foam spots are commercially unacceptable on workpieces which are only to be coated once.
A variety of techniques have been used to solve the foregoing problems. For example, the paint bath can be sprayed into the part of the workpiece corresponding to the air pocket before the workpiece is lowered into the paint bath. Eductors or nozzles disposed in the coating tank can force the air bubble out by forcibly circulating the paint. Guide bars can also be installed in the paint bath tank to tilt (roll) the hanger in order to "burp" the air bubble out of the work piece. These methods have proven unsatisfactory in practice because they are generally not useful with a hanger for holding multiple workpieces, and also because intank modifications such as guide bars interfere with painting operations for other products of different shapes. The present invention solves the problems inherent in these known methods by incorporating means for removing a trapped air bubble into the hanger itself, as will be described hereafter.
A wide variety of systems are known for transporting treated workpieces, particularly painted or surface-treated can-shaped workpieces. Hamal U.S. Pat. No. 2,746,882 issued May 22, 1956, Dowling U.S. Pat. No. 3,365,158 issued Jan. 23, 1968, Monk U.S. Pat. No. 4,187,801 issued Feb. 12, 1980 and Oswald U.S. Pat. No. 4,213,418 issued July 22, 1980 are illustrative of such systems. Racks or hangers including a frame or support having a plurality of pins, arms or hooks for supporting objects are also widely known. The following U.S. patents are representative of such devices: Walther U.S. Pat. No. 3,252,583 issued May 24, 1966, Woorhies U.S. Pat. No. 3,642,147 issued Feb. 15, 1972, Scholl U.S. Pat. No. 3,780,875 issued Dec. 25, 1973, Lazarus U.S. Pat. No. 4,290,531 issued Sept. 22, 1981, and Turner U.S. Pat. No. 4,461,386 issued July 24, 1984. In particular, the following U.S. patents describe racks for use in electropainting wherein a plurality of hooks are mounted on a electrically conductive frame: Davitz U.S. Pat. Nos. 4,097,359 issued June 27, 1978, 4,217,853, issued Aug. 19, 1980, and 4,243,146 issued Jan. 6, 1981. None of these prior hangers and racks provide a convenient solution to the problem of electropainting workpieces which tend to capture air bubbles.